# Thread: real solutions and mathematical model

1. ## real solutions and mathematical model

I'm having trouble with a few of my homework problems this evening. *sigh*

1. Find all real solutions of x4 - 7x3 + 7x² + 21x - 30 = 0

and

2. Find a mathematical model representing the statement. (In each case determine the constant of proportionality.)
F varies directly as x and inversely as y, and F = 500 when x = 15 and y = 2

I thought number one would be easy. I guess I was wrong.

2. Originally Posted by uglygreencouch
I'm having trouble with a few of my homework problems this evening. *sigh*

1. Find all real solutions of x4 - 7x3 + 7x² + 21x - 30 = 0

.
Do rational roots theorem and see what you get.

3. Originally Posted by uglygreencouch
I'm having trouble with a few of my homework problems this evening. *sigh*

1. Find all real solutions of x4 - 7x3 + 7x² + 21x - 30 = 0
as TPH says, use the rational roots theorem. the factor theorem will also come in handy, you can look these up on wikipedia. if you still have problems, just say so

2. Find a mathematical model representing the statement. (In each case determine the constant of proportionality.)
F varies directly as x and inversely as y, and F = 500 when x = 15 and y = 2
F varies directly as x means $F = k_1x$ for some constant $k_1$.

when F = 500, x = 15, use this to find $k_1$ and then plug it into the original equation to find the model.

do a similar thing with the second case, just note that F varies inversely as y means $F = \frac {k_2}y$ for some constant $k_2$

4. thanks guys